Sensor element

ABSTRACT

A sensor element includes: an element base including: a ceramic body made of an oxygen-ion conductive solid electrolyte, and having a gas inlet at one end portion thereof; at least two internal chambers located inside the ceramic body, and communicating with the gas inlet under predetermined diffusion resistance; an electrochemical pump cell including an electrode located on an outer surface of the ceramic body, an electrode facing the internal chambers, and solid electrolytes located therebetween; and a heater buried in the ceramic body; and a porous leading-end protective layer surrounding a first range at least including a part from a leading end surface to two internal chambers close to the gas inlet of the element base. A single heat insulating space is interposed between the leading-end protective layer and a portion of the element base in which the two internal chambers are located.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese applicationJP2018-146685, filed on Aug. 3, 2018, the contents of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a gas sensor detecting a predeterminedgas component in a measurement gas, and, in particular, to aconfiguration for preventing water-induced cracking of a sensor elementincluded in the gas sensor.

Description of the Background Art

As a gas sensor for determining concentration of a desired gas componentin a measurement gas, a gas sensor that includes a sensor element madeof an oxygen-ion conductive solid electrolyte, such as zirconia (ZrO₂),and including some electrodes on the surface and the inside thereof hasbeen widely known. Such a sensor element includes a protective layerformed of a porous body (porous protective layer) to prevent cracking ofthe sensor element (more particularly, an element base) occurring due tothermal shock caused by adherence of water droplets, which is so-calledwater-induced cracking. The extent of the effect of preventing thewater-induced cracking is also referred to as water resistance.

As such a sensor element, a sensor element including protective layersprovided on opposite main surfaces of an elongated planar element base,and further including a porous protective layer provided to a leadingend portion has already been known (see Japanese Patent ApplicationLaid-Open No. 2016-48230, for example).

A sensor element including a porous protective layer formed at a leadingend portion of an elongated planar element base to have a space betweenthe layer and an element has also already been known (see JapanesePatent Application Laid-Open No. 2016-188853 and Japanese PatentApplication Laid-Open No. 2015-87161, for example).

Japanese Patent Application Laid-Open No. 2016-48230 discloses thatforming the porous protective layer in a region, of the leading endportion of the sensor element, in a temperature state of 500° C. or morewhen the gas sensor is in use while not forming the porous protectivelayer in a region in a temperature state of 300° C. or less when the gassensor is in use can reduce power consumption and a waiting time untildetection due to reduction in area of formation of the porous protectivelayer, and can achieve suppression of cracking due to improvement inwater resistance.

The sensor element according to Japanese Patent Application Laid-OpenNo. 2016-48230, however, does not necessarily have sufficient waterresistance, and is subject to water-induced cracking in a case where theamount of water exposure is large.

Japanese Patent Application Laid-Open No. 2016-188853 discloses a sensorelement including a porous protective layer adhering to one leading endsurface of an element base while having a space between the layer and aside surface perpendicular to the leading end surface of the elementbase. This configuration is effective in terms of weakening thermalconduction from the porous protective layer to the element base.

Japanese Patent Application Laid-Open No. 2016-188853 schematicallydiscloses a positional relationship between a portion of the sensorelement in which the temperature becomes high when the gas sensor is inuse and the space, but fails to particularly disclose the relationshipbetween temperature distribution of the sensor element when the gassensor is in use and the shape and placement of the porous protectivelayer. Water resistance may thus not sufficiently be secured in a casewhere the configuration disclosed in Japanese Patent ApplicationLaid-Open No. 2016-188853 is adopted. In the sensor element disclosed inJapanese Patent Application Laid-Open No. 2016-188853, the porousprotective layer is provided to be directly joined to a dense solidelectrolyte layer. With the configuration disclosed in Japanese PatentApplication Laid-Open No. 2016-188853, adhesion of the porous protectivelayer to the side surface of the element base is not necessarilysecured, and delamination and, further, detachment of the porousprotective layer may occur. The occurrence of such delamination anddetachment is not preferable as it impairs water resistance of thesensor element as originally assumed.

On the other hand, forming a space only in a corner portion at one endportion of the sensor element as disclosed in Japanese PatentApplication Laid-Open No. 2015-87161 is not preferable as water-inducedcracking may occur in a portion in which the temperature becomes highwhen the gas sensor is in use but no space is provided.

SUMMARY

The present invention relates to a gas sensor detecting a predeterminedgas component in a measurement gas, and is, in particular, directed toprevention of water-induced cracking of a sensor element included in thegas sensor.

According to the present invention, a sensor element for a gas sensordetecting a predetermined gas component in a measurement gas includes:an element base including: an elongated planar ceramic body made of anoxygen-ion conductive solid electrolyte, and having a gas inlet at oneend portion thereof; at least two internal chambers located inside theceramic body, and communicating with the gas inlet under predetermineddiffusion resistance; at least two electrochemical pump cells includingan outer pump electrode located on an outer surface of the ceramic body,at least two inner pump electrodes located to face each of the at leasttwo internal chambers, and solid electrolytes located between the outerpump electrode and each of the at least two inner pump electrodes, theat least two electrochemical pump cells pumping in and out oxygenbetween each of the at least two internal chambers and an outside; and aheater buried in a predetermined range on a side of the one end portionof the ceramic body; and a first leading-end protective layer beingporous, and surrounding a first range at least including a part from aleading end surface to two internal chambers close to the gas inlet ofthe element base on the side of the one end portion, wherein a singleheat insulating space is interposed between the first leading-endprotective layer and a portion of the element base in which at least thetwo internal chambers close to the gas inlet are located.

The sensor element having greater water resistance than that of aconventional sensor element can thereby be achieved.

The sensor element according to the present invention preferably furtherincludes a second leading-end protective layer being porous, having alarger porosity than the first leading-end protective layer, and locatedon a whole side surface of the element base at least in the first range,wherein an end portion of the first leading-end protective layeropposite the one end portion is a fixed portion in which the firstleading-end protective layer is fixed to the second leading-endprotective layer.

The sensor element in which delamination and, further, detachment of thefirst leading-end protective layer have suitably been suppressed canthereby be achieved.

More preferably, the first leading-end protective layer is further fixedto the leading end surface of the element base on the side of the oneend portion, and the single heat insulating space is present onlybetween the first leading-end protective layer and the side surface ofthe element base in the first range surrounded by the first leading-endprotective layer.

In this case, the sensor element in which delamination and, further,detachment of the first leading-end protective layer have more surelyand suitably been suppressed compared with a case where the firstleading-end protective layer is fixed to the second leading-endprotective layer only on the side surface of the element base canthereby be achieved.

It is thus an object of the present invention to provide a sensorelement including a porous protective layer on a side of one end portionat which an inlet for a measurement gas is provided, and having greaterwater resistance than that of a conventional sensor element.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic external perspective view of a sensor element 10according to a first embodiment;

FIG. 2 is a schematic diagram illustrating a configuration of a gassensor 100 including a sectional view taken along a longitudinaldirection of the sensor element 10;

FIG. 3 is a diagram for more particularly describing specific placementlocations of an outer leading-end protective layer 2 and a heatinsulating space 4, and the significance thereof;

FIG. 4 illustrates an example of the relationship between aconfiguration of the sensor element 10 and a temperature profile of thesensor element 10 when the sensor element 10 is heated by a heater 150in accordance with a predetermined control condition when the sensorelement 10 is in use;

FIG. 5 is a flowchart of processing at the manufacture of the sensorelement 10;

FIGS. 6A to 6F schematically illustrate specific procedures for formingthe heat insulating space 4 and the outer leading-end protective layer2;

FIG. 7 is a sectional view taken along the longitudinal direction of thesensor element 20 according to a second embodiment;

FIG. 8 is a diagram for more particularly describing specific placementlocations of an outer leading-end protective layer 12 and the heatinsulating space 4, and the significance thereof; and

FIG. 9 illustrates an example of the relationship between aconfiguration of a sensor element 20 and a temperature profile of thesensor element 20 when the sensor element 20 is heated by the heater 150in accordance with a predetermined control condition when the sensorelement 20 is in use.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

<Overview of Sensor Element and Gas Sensor>

FIG. 1 is a schematic external perspective view of a sensor element (gassensor element) 10 according to a first embodiment of the presentinvention. FIG. 2 is a schematic diagram illustrating a configuration ofa gas sensor 100 including a sectional view taken along a longitudinaldirection of the sensor element 10. The sensor element 10 is a maincomponent of the gas sensor 100 detecting a predetermined gas componentin a measurement gas, and measuring concentration thereof. The sensorelement 10 is a so-called limiting current gas sensor element.

In addition to the sensor element 10, the gas sensor 100 mainly includesa pump cell power supply 30, a heater power supply 40, and a controller50.

As illustrated in FIG. 1, the sensor element 10 has a configuration inwhich one end portion of an elongated planar element base 1 is coveredwith a porous outer leading-end protective layer (first leading-endprotective layer) 2.

As illustrated in FIG. 2, the element base 1 includes an elongatedplanar ceramic body 101 as a main structure, and main surface protectivelayers 170 are provided on two main surfaces of the ceramic body 101,and, further, inner leading-end protective layers (second leading-endprotective layers) 180 are provided outside four side surfaces (on anouter periphery other than a leading end surface 101 e) on a side of theone end portion. In addition, the sensor element 10 includes the outerleading-end protective layer 2 provided further outside the innerleading-end protective layers 180. The outer leading-end protectivelayer 2, however, is provided so that a space (heat insulating space) 4is interposed between the outer leading-end protective layer 2 and theelement base 1. The four side surfaces of the sensor element 10 (or theelement base 1, or the ceramic body 101) other than opposite endsurfaces in the longitudinal direction thereof are hereinafter simplyreferred to as side surfaces of the sensor element 10 (or the elementbase 1, or the ceramic body 101). The leading end surface 101 e of theceramic body 101 is also referred to as the leading end surface 101 e ofthe element base 1.

The ceramic body 101 is made of ceramic containing, as a main component,zirconia (yttrium stabilized zirconia), which is an oxygen-ionconductive solid electrolyte. Various components of the sensor element10 are provided outside and inside the ceramic body 101. The ceramicbody 101 having the configuration is dense and airtight. Theconfiguration of the sensor element 10 illustrated in FIG. 2 is just anexample, and a specific configuration of the sensor element 10 is notlimited to this configuration.

The sensor element 10 illustrated in FIG. 2 is a so-called serialthree-chamber structure type gas sensor element including a firstinternal chamber 102, a second internal chamber 103, and a thirdinternal chamber 104 inside the ceramic body 101. That is to say, in thesensor element 10, the first internal chamber 102 communicates, througha first diffusion control part 110 and a second diffusion control part120, with a gas inlet 105 opening to the outside on a side of one endportion E1 of the ceramic body 101 (to be precise, communicating withthe outside through the outer leading-end protective layer 2), thesecond internal chamber 103 communicates with the first internal chamber102 through a third diffusion control part 130, and the third internalchamber 104 communicates with the second internal chamber 103 through afourth diffusion control part 140. A path from the gas inlet 105 to thethird internal chamber 104 is also referred to as a gas distributionpart. In the sensor element 10 according to the present embodiment, thedistribution part is provided straight along the longitudinal directionof the ceramic body 101.

The first diffusion control part 110, the second diffusion control part120, the third diffusion control part 130, and the fourth diffusioncontrol part 140 are each provided as two slits vertically arranged inFIG. 2. The first diffusion control part 110, the second diffusioncontrol part 120, the third diffusion control part 130, and the fourthdiffusion control part 140 provide predetermined diffusion resistance toa measurement gas passing therethrough. A buffer space 115 having aneffect of buffering pulsation of the measurement gas is provided betweenthe first diffusion control part 110 and the second diffusion controlpart 120.

An external pump electrode 141 is provided on an outer surface of theceramic body 101, and an internal pump electrode 142 is provided in thefirst internal chamber 102. Furthermore, an auxiliary pump electrode 143is provided in the second internal chamber 103, and a measurementelectrode 145 is provided in the third internal chamber 104. Inaddition, a reference gas inlet 106 which communicates with the outsideand through which a reference gas is introduced is provided on a side ofthe other end portion E2 of the ceramic body 101, and a referenceelectrode 147 is provided in the reference gas inlet 106.

In a case where a target of measurement of the sensor element 10 is NOxin the measurement gas, for example, concentration of a NOx gas in themeasurement gas is calculated by a process as described below.

First, the measurement gas introduced into the first internal chamber102 is adjusted to have an approximately constant oxygen concentrationby a pumping action (pumping in or out of oxygen) of a main pump cellP1, and then introduced into the second internal chamber 103. The mainpump cell P1 is an electrochemical pump cell including the external pumpelectrode 141, the internal pump electrode 142, and a ceramic layer 101a that is a portion of the ceramic body 101 existing between theseelectrodes. In the second internal chamber 103, oxygen in themeasurement gas is pumped out of the element by a pumping action of anauxiliary pump cell P2 that is also an electrochemical pump cell, sothat the measurement gas is in a sufficiently low oxygen partialpressure state. The auxiliary pump cell P2 includes the external pumpelectrode 141, the auxiliary pump electrode 143, and a ceramic layer 101b that is a portion of the ceramic body 101 existing between theseelectrodes.

The external pump electrode 141, the internal pump electrode 142, andthe auxiliary pump electrode 143 are each formed as a porous cermetelectrode (e.g., a cermet electrode made of ZrO₂ and Pt that contains Auof 1%). The internal pump electrode 142 and the auxiliary pump electrode143 to be in contact with the measurement gas are each formed using amaterial having weakened or no reducing ability with respect to a NOxcomponent in the measurement gas.

NOx in the measurement gas caused by the auxiliary pump cell to be inthe low oxygen partial pressure state is introduced into the thirdinternal chamber 104, and reduced or decomposed by the measurementelectrode 145 provided in the third internal chamber 104. Themeasurement electrode 145 is a porous cermet electrode also functioningas a NOx reduction catalyst that reduces NOx existing in the atmospherein the third internal chamber 104. During the reduction ordecomposition, a potential difference between the measurement electrode145 and the reference electrode 147 is maintained constant. Oxygen ionsgenerated by the above-mentioned reduction or composition are pumped outof the element by a measurement pump cell P3. The measurement pump cellP3 includes the external pump electrode 141, the measurement electrode145, and a ceramic layer 101 c that is a portion of the ceramic body 101existing between these electrodes. The measurement pump cell P3 is anelectrochemical pump cell pumping out oxygen generated by decompositionof NOx in the atmosphere around the measurement electrode 145.

Pumping (pumping in or out of oxygen) of the main pump cell P1, theauxiliary pump cell P2, and the measurement pump cell P3 is achieved,under control performed by the controller 50, by the pump cell powersupply (variable power supply) 30 applying voltage necessary for pumpingacross electrodes included in each of the pump cells. In a case of themeasurement pump cell P3, voltage is applied across the external pumpelectrode 141 and the measurement electrode 145 so that the potentialdifference between the measurement electrode 145 and the referenceelectrode 147 is maintained at a predetermined value. The pump cellpower supply 30 is typically provided for each pump cell.

The controller 50 detects a pump current Ip2 flowing between themeasurement electrode 145 and the external pump electrode 141 inaccordance with the amount of oxygen pumped out by the measurement pumpcell P3, and calculates a NOx concentration in the measurement gas basedon a linear relationship between a current value (NOx signal) of thepump current Ip2 and the concentration of decomposed NOx.

The gas sensor 100 preferably includes a plurality of electrochemicalsensor cells, which are not illustrated, detecting the potentialdifference between each pump electrode and the reference electrode 147,and each pump cell is controlled by the controller 50 based on a signaldetected by each sensor cell.

In the sensor element 10, a heater 150 is buried in the ceramic body101. The heater 150 is provided, below the gas distribution part in FIG.2, over a range from the vicinity of the one end portion E1 to at leasta location of formation of the measurement electrode 145 and thereference electrode 147. The heater 150 is provided mainly to heat thesensor element 10 to enhance oxygen-ion conductivity of the solidelectrolyte forming the ceramic body 101 when the sensor element 10 isin use. More particularly, the heater 150 is provided to be surroundedby an insulating layer 151.

The heater 150 is a resistance heating body made, for example, ofplatinum. The heater 150 generates heat by being powered from the heaterpower supply 40 under control performed by the controller 50.

The sensor element 10 according to the present embodiment is heated bythe heater 150 when being in use so that the temperature at least in arange from the first internal chamber 102 to the second internal chamber103 becomes 500° C. or more. In some cases, the sensor element 10 isheated so that the temperature of the gas distribution part as a wholefrom the gas inlet 105 to the third internal chamber 104 becomes 500° C.or more. These are to enhance the oxygen-ion conductivity of the solidelectrolyte forming each pump cell and to desirably demonstrate theability of each pump cell. In this case, the temperature in the vicinityof the first internal chamber 102, which becomes the highesttemperature, becomes approximately 700° C. to 800° C.

In the following description, from among the two main surfaces of theceramic body 101, a main surface (or an outer surface of the sensorelement 10 having the main surface) which is located on an upper side inFIG. 2 and on a side where the main pump cell P1, the auxiliary pumpcell P2, and the measurement pump cell P3 are mainly provided is alsoreferred to as a pump surface, and a main surface (or an outer surfaceof the sensor element 10 having the main surface) which is located on alower side in FIG. 2 and on a side where the heater 150 is provided isalso referred to as a heater surface. In other words, the pump surfaceis a main surface closer to the gas inlet 105, the three internalchambers, and the pump cells than to the heater 150, and the heatersurface is a main surface closer to the heater 150 than to the gas inlet105, the three internal chambers, and the pump cells.

A plurality of electrode terminals 160 are provided on the respectivemain surfaces of the ceramic body 101 on the side of the other endportion E2 to establish electrical connection between the sensor element10 and the outside. These electrode terminals 160 are electricallyconnected to the above-mentioned five electrodes, opposite ends of theheater 150, and a lead for detecting heater resistance, which is notillustrated, through leads provided inside the ceramic body 101, whichare not illustrated, to have a predetermined correspondencerelationship. Application of a voltage from the pump cell power supply30 to each pump cell of the sensor element 10 and heating by the heater150 by being powered from the heater power supply 40 are thus performedthrough the electrode terminals 160.

The sensor element 10 further includes the above-mentioned main surfaceprotective layers 170 (170 a, 170 b) on the pump surface and the heatersurface of the ceramic body 101. The main surface protective layers 170are layers made of alumina, having a thickness of approximately 5 μm to30 μm, and including pores with a porosity of approximately 20% to 40%,and are provided to prevent adherence of any foreign matter and poisonedsubstances to the main surfaces (the pump surface and the heatersurface) of the ceramic body 101 and the external pump electrode 141provided on the pump surface. The main surface protective layer 170 a onthe pump surface thus functions as a pump electrode protective layer forprotecting the external pump electrode 141.

In the present embodiment, the porosity is obtained by applying a knownimage processing method (e.g., binarization processing) to a scanningelectron microscope (SEM) image of an evaluation target.

The main surface protective layers 170 are provided over substantiallyall of the pump surface and the heater surface except that the electrodeterminals 160 are partially exposed in FIG. 2, but this is just anexample. The main surface protective layers 170 may locally be providedin the vicinity of the external pump electrode 141 on the side of theone end portion E1 compared with the case illustrated in FIG. 2. Themain surface protective layers 170, however, are provided, on the pumpsurface and the heater surface, at least in a range in which the innerleading-end protective layers 180 are formed.

On the side of the one end portion E1 of the element base 1 included inthe sensor element 10, the above-mentioned inner leading-end protectivelayers 180 are further provided outside the side surfaces (on the outerperiphery other than the leading end surface 101 e on which the gasinlet 105 is provided). The inner leading-end protective layers 180 areporous layers made of alumina, having a relatively large porosity of 30%to 50%, and having a thickness of 20 μm to 50 μm.

The inner leading-end protective layers 180 have a role of preventingpoisoning and exposure to water of the sensor element 10 along with theouter leading-end protective layer 2 and the main surface protectivelayers 170. For example, the inner leading-end protective layers 180have higher heat insulating properties than those of the outerleading-end protective layer 2 and the main surface protective layers170 as they have a larger porosity, and this contributes to improvementin water resistance of the sensor element 10.

The inner leading-end protective layers 180 also have a role asunderlying layers when the outer leading-end protective layer 2 isformed with respect to the element base 1. It is only required that theinner leading-end protective layers 180 be formed, on the side surfacesof the element base 1, at least in a range surrounded by the outerleading-end protective layer 2.

<Outer Leading-End Protective Layer and Heat Insulating Space>

In the sensor element 10, the outer leading-end protective layer 2 thatis a porous layer made of alumina having a purity of 99.0% or more isprovided around an outermost periphery in a predetermined range from theone end portion E1 of the element base 1 having a configuration asdescribed above.

The outer leading-end protective layer 2, however, is provided tosurround the one end portion E1 of the element base 1 so that the space(heat insulating space) 4 is interposed between the outer leading-endprotective layer 2 and the element base 1 as can be seen from FIG. 2.However, the outer leading-end protective layer 2 is fixed (joined) tothe element base 1 only in a portion where the outer leading-endprotective layer 2 is in contact with the inner leading-end protectivelayers 180, which is provided to be separated from the leading endportion by a predetermined distance. The heat insulating space 4 isseparated from the outside only by the outer leading-end protectivelayer 2, which is the porous layer, and is thus not an enclosed space.Thus, gas flows in and out between the heat insulating space 4 and theoutside at all times. Introduction of the measurement gas into theelement base 1 (ceramic body 101) through the gas inlet 105 is naturallyperformed without any problems.

In the following description, a part of the heat insulating space 4along the side surfaces of the element base 1 is referred to as a firstspace 4 a, and a part along the leading end surface 101 e is referred toas a second space 4 b. In particular, a part of the first space 4 aalong the pump surface is also referred to as a pump surface-side space4 a 1, and a part along the heater surface is also referred to as aheater surface-side space 4 a 2. The first space 4 a and the secondspace 4 b, however, are not independent of each other, and arecontiguous to each other. That is to say, the heat insulating space 4absolutely forms one space as a whole.

A portion of the outer leading-end protective layer 2 being in contactwith the inner leading-end protective layers 180 is referred to as afixed portion 201, a portion of the outer leading-end protective layer 2surrounding the side surfaces of the element base 1 to form the firstspace 4 a with the element base 1 is referred to as a side surfaceportion 202, and a portion of the outer leading-end protective layer 2surrounding the leading end surface 101 e of the element base 1 to formthe second space 4 b with the element base 1 is referred to as an endsurface portion 203.

That is to say, the outer leading-end protective layer 2 is fixed to theelement base 1 (specifically, to the inner lading end protective layers180) only in the fixed portion 201 having a band shape sequentiallyalong the side surfaces of the element base 1. A portion in which thefixed portion 201 and the element base 1 (inner leading-end protectivelayers 180) are in contact with each other is preferably 10% or more, inarea, of a total range in which the outer leading-end protective layer 2surrounds the element base 1. In this case, stable fixing to the elementbase 1 is achieved. An area ratio of the fixed portion 201 (a fixed arearatio) of less than 10% is not preferable as sufficient adhesionstrength cannot be obtained. The upper limit of the fixed area ratio isdetermined in accordance with a minimum formation range of the heatinsulating space 4 meeting a desired condition, and a fixed area ratioof 50% is sufficient in practical use.

The outer leading-end protective layer 2 is provided to surround aportion of the element base 1 in which the temperature becomes high whenthe gas sensor 100 is in use to thereby obtain water resistance in theportion. The outer leading-end protective layer 2 suppresses theoccurrence of cracking (water-induced cracking) of the element base 1due to thermal shock caused by local temperature reduction upon directexposure of the portion to water. The reason why the heat insulatingspace 4 is interposed between the outer leading-end protective layer 2and the element base 1 is that, even if the outer leading-end protectivelayer 2 is exposed to water to cause the local temperature reduction,the interposed space having a large heat capacity suitably suppressesthe occurrence of the water-induced cracking caused by the action of thethermal shock on the element base 1.

The outer leading-end protective layer 2 is formed to have a thicknessof 150 μm or more to 600 μm or less. The thickness of the outerleading-end protective layer 2 hereinafter refers to the thickness ofthe side surface portion 202 and the end surface portion 203. The sidesurface portion 202 and the end surface portion 203, however, may nothave the same thickness. On the other hand, the thickness of the fixedportion 201 may have a greater value than that of the thickness of theside surface portion 202 as long as the fixed portion 201 does notprotrude farther from the side surface portion 202 in an elementthickness direction and an element width direction of the sensor element10.

A thickness of the outer leading-end protective layer 2 of less than 150μm is not preferable as, due to reduction in strength of the outerleading-end protective layer 2 itself, resistance to the thermal shockis reduced and water resistance is reduced, and, further, resistance toshock acting due to vibration or other factors is reduced. On the otherhand, a thickness of the outer leading-end protective layer 2 of morethan 600 μm is not preferable as, due to an increase in heat capacity ofthe outer leading-end protective layer 2, power consumption increaseswhen the heater 150 performs heating, and, due to an increase in gasdiffusion time, responsiveness of the sensor element 10 is degraded.

The outer leading-end protective layer 2 is provided so that the heatinsulating space 4 has a thickness (the distance between the elementbase 1 and the outer leading-end protective layer 2) of 30 μm or moreand 150 μm or less.

A thickness of the heat insulating space 4 of less than 30 μm is notpreferable as a heat insulating effect is not suitably obtained, andwater resistance is reduced. On the other hand, a thickness of the heatinsulating space 4 of more than 150 μm is not preferable as stressacting on the fixed portion 201 of the outer leading-end protectivelayer 2 increases, and delamination and, further, detachment of theouter leading-end protective layer 2 are more likely to occur.

The outer leading-end protective layer 2 preferably has a smallerporosity than the inner leading-end protective layers 180.

When the inner leading-end protective layers 180 have a larger porosity,a so-called anchoring effect acts between the fixed portion 201 of theouter leading-end protective layer 2 and the inner leading-endprotective layers 180 as the underlying layers. Due to the action of theanchoring effect, in the sensor element 10, delamination of the outerleading-end protective layer 2 from the element base 1 caused by adifference in coefficient of thermal expansion between the outerleading-end protective layer 2 and the element base 1 is more suitablysuppressed when the sensor element 10 is in use.

The main surface protective layers 170 are made of alumina as with theinner leading-end protective layers 180, but have a smaller porosity anda smaller thickness than the inner leading-end protective layers 180,and thus, if the inner leading-end protective layers 180 are omitted toprovide the outer leading-end protective layer 2 directly on the mainsurface protective layers 170, such an effect of mitigating thedifference in thermal expansion as is obtained with the innerleading-end protective layers 180 cannot highly be expected.

The inner leading-end protective layers 180 adjacent to the heatinsulating space 4 are formed to have a relatively large porosity of 30%to 50% as described above, and thus have a larger heat capacity than theouter leading-end protective layer 2 and the main surface protectivelayers 170, although it is smaller than heat capacity of the heatinsulating space 4. The presence of the inner leading-end protectivelayers 180 contributes to suppression of the water-induced cracking aswith the heat insulating space 4.

The outer leading-end protective layer 2 more preferably has a porosityof 15% to 30%. A porosity of the outer leading-end protective layer 2 ofless than 15% is not preferable as a risk of clogging with poisonedsubstances increases, and responsiveness of the sensor element 10 isdegraded. On the other hand, a porosity of more than 30% is notpreferable as the strength of the outer leading-end protective layer 2is not secured.

Furthermore, the outer leading-end protective layer 2 is provided sothat the fixed portion 201 of the outer leading-end protective layer 2and the inner leading-end protective layers 180 make a predetermined endportion angle (acute angle) θ on a side of an end portion of the heatinsulating space 4. The end portion angle θ is preferably 5° to 15°. Inthis case, adhesion of the outer leading-end protective layer 2 to theinner leading-end protective layers 180 increases. The end portion angleθ may not necessarily be identical on each side surface, and, forexample, an end portion angle θ1 on the pump surface and an end portionangle θ2 on the heater surface may have different values.

FIG. 3 is a diagram for more particularly describing specific placementlocations of the outer leading-end protective layer 2 and the heatinsulating space 4, and the significance thereof. As illustrated in FIG.3, in the element base 1, three zones, that is, zones A, B, and C areconceptually defined in a longitudinal direction of the element.Placement of the outer leading-end protective layer 2 and the heatinsulating space 4 is determined based on these zones.

The zone A is a region heated by the heater 150 to a temperature of 500°C. or more when the gas sensor 100 is in use. As described above, whenthe gas sensor 100 is in use, the sensor element 10 is heated by theheater 150 so that the temperature at least in the range from the firstinternal chamber 102 to the second internal chamber 103 becomes 500° C.or more. The range thus belongs to the zone A at any time. FIG. 3illustrates a case where the zone A substantially coincides with aportion including the gas distribution part from the gas inlet 105 tothe third internal chamber 104 in the longitudinal direction of theelement base 1.

In contrast, the zone B is a region starting at an end portion of thefixed portion 201 in which the outer leading-end protective layer 2 isfixed to the inner leading-end protective layers 180 on the side of theone end portion E1, and ending at the other end portion E2 of theelement base 1. The zone B is maintained at 500° C. or less when the gassensor 100 is in use during which the sensor element 10 is heated by theheater 150. More specifically, in the zone B, the temperature decreaseswith increasing distance from the one end portion E1 of the element base1, and a region in which the temperature becomes 500° C. is limited tothe vicinity of the boundary with the zone C or A.

The zone C is a region between the zones A and B in the longitudinaldirection of the element base 1. The zone C, however, is not necessarilyrequired, and the zones A and B may be adjacent to each other.

In the sensor element 10 of the gas sensor 100 according to the presentembodiment, since the fixed portion 201 in which the outer leading-endprotective layer 2 is fixed to the inner leading-end protective layers180 is included in the zone B, the heat insulating space 4 (the firstspace 4 a and the second space 4 b) is inevitably present at leastaround a portion of the element base 1 belonging to the zone A,including the leading end portion.

In other words, a portion of the element base 1 heated to a hightemperature of 500° C. or more when the gas sensor 100 is in use is notin contact with the outer leading-end protective layer 2, and the heatinsulating space 4 is surely provided around the portion. When the gassensor 100 is in use, the side surface portion 202 and the end surfaceportion 203 of the outer leading-end protective layer 2 are also heatedto a high temperature of 500° C. or more.

In practical use of the gas sensor 100 including the sensor element 10in which the outer leading-end protective layer 2 and the heatinsulating space 4 are provided in a manner as described above, thesensor element 10 is heated by the heater 150 so that a temperatureprofile in which the temperature is 500° C. or more in the zone A whilethe temperature is 500° C. or less in the zone B is achieved.

In this heating situation, once water vapor included in the measurementgas adheres, as water droplets, to the side surface portion 202 or theend surface portion 203 of the outer leading-end protective layer 2belonging to the zone A, that is, the portion of the sensor element 10heated to a high temperature of 500° C. or more is exposed to water,local and abrupt temperature reduction occurs in the adherence portion(water-exposed portion). The side surface portion 202 and the endsurface portion 203 of the outer leading-end protective layer 2,however, are not in contact with the element base 1, and the heatinsulating space 4 (the first space 4 a and the second space 4 b) havinga large heat capacity is interposed between them, and thus thermal shockcaused by the temperature reduction in the water-exposed portion doesnot occur in the element base 1. This means that the occurrence of thewater-induced cracking of the sensor element 10 is suitably prevented byusing the configuration in which the porous outer leading-end protectivelayer 2 is provided in the portion in which the temperature becomes 500°C. or more when the gas sensor 100 is in use, and the heat insulatingspace 4 is interposed between the outer leading-end protective layer 2and the element base 1 as in the gas sensor 100 according to the presentembodiment.

It is confirmed in advance that, even if water droplets adhere to aportion in which the temperature is 500° C. or less, abrupt temperaturereduction hardly occurs, and thus thermal shock that can cause thewater-induced cracking hardly occurs.

FIG. 4 illustrates an example of the relationship between theconfiguration of the sensor element 10 and the temperature profile ofthe sensor element 10 when the sensor element 10 is heated by the heater150 in accordance with a predetermined control condition when the sensorelement 10 is in use. The temperature profile shown in FIG. 4 isobtained by measuring the surface temperature on the pump surface of thesensor element 10 along the longitudinal direction of the element, andplotting it with the location of the leading end surface 101 e on theside of the one end portion E1 as the origin. Thermography is used tomeasure the surface temperature.

In the example illustrated in FIG. 4, a range extending from the leadingend of the element (one end portion E1) by a distance L1 is the zone A,and a range separated from the leading end of the element by a distanceL2 or more is the zone B.

If the control condition of the heater 150 is changed, the temperatureprofile of the sensor element 10 changes. The properties of the sensorelement 10, however, depend on the heating state, and thus the heater150 typically performs heating so that the same temperature profile isobtained at all times, based on one control condition fixedly set inadvance at the time of manufacture (typically, further, to exert theproperties of the element as much as possible). The sensor element 10 isthus heated so that the steady temperature profile is obtained.Accordingly, the portion of the element base 1 heated to a temperatureof 500° C. or more is the same at all times, and the ranges of the zonesA, B, and C may be considered to be fixed in each sensor element 10.

Thus, having only to specify the zones and provide the outer leading-endprotective layer 2 so that the heat insulating space 4 is formed inaccordance with the ranges of the zones at the manufacture of the sensorelement 10, the heat insulating space 4 comes to exist around the region(i.e., the zone A) every time heated by the heater 150 to a temperatureof 500° C. or more during use after the manufacture.

As for numerous sensor elements 10 manufactured under the samecondition, such as sensor elements 10 industrially produced in largequantities, if the sensor elements 10 are heated by the heaters 150under the same control condition, the temperature profiles obtained fromthe sensor elements 10 are approximately the same as long as they aremanufactured properly. Thus, having only to specify the temperatureprofile for a sensor element 10 extracted as a sample, and to demarcatethe ranges of the zones A, B, and C based on the temperature profile, acondition for forming the outer leading-end protective layer 2 can bedetermined, based on the results, for all sensor elements 10manufactured under the same condition without actually specifying thetemperature profiles for all the sensor elements 10. That is to say, itis not necessary to actually obtain the temperature profiles for all thesensor elements 10, and demarcate the ranges of the zones A, B, and Cbased on the results.

In other words, it can be said that, for the sensor elements 10manufactured under the same condition as described above, a region (aregion to be coped with water-induced cracking) of the element base 1 isspecified in advance in accordance with setting of the control conditionof the heater 150, which is a region where the water-induced crackingmay occur upon receipt of thermal shock caused by adherence of waterdroplets during use, and thus any coping with the water-induced crackingis needed. In the case of FIGS. 3 and 4, the zone A corresponds to theregion. It can be said that the outer leading-end protective layer 2surrounds a predetermined range of the element base 1 on the side of theone end portion E1 so that the heat insulating space 4 is interposedbetween the region to be coped with water-induced cracking and the outerleading-end protective layer 2. It can also be said that, in this case,the outer leading-end protective layer 2 is fixed to the element base 1(to the inner leading-end protective layers 180) in a region specifiedin advance as a region (water-induced cracking not occurring region) inwhich the water-induced cracking does not occur during use. In the caseof FIGS. 3 and 4, the zone B corresponds to the region.

As described above, according to the present embodiment, the outerleading-end protective layer as the porous layer is provided at leastaround the region to be coped with water-induced cracking specified inadvance and including the range from the first internal chamber to thesecond internal chamber of the element base of the sensor elementincluded in the gas sensor so that the heat insulating space isinterposed between the outer leading-end protective layer and theelement base. The sensor element having greater water resistance thanthat of a conventional sensor element can thereby be achieved.Furthermore, the inner leading-end protective layers having a largerporosity than the outer leading-end protective layer are provided on theouter periphery of the element base, and the outer leading-endprotective layer is fixed to the inner leading-end protective layers inthe water-induced cracking not occurring region specified in advance.Delamination and, further, detachment of the outer leading-endprotective layer can thereby suitably be suppressed.

<Process of Manufacturing Sensor Element>

One example of a process of manufacturing the sensor element 10 having aconfiguration and features as described above will be described next.FIG. 5 is a flowchart of processing at the manufacture of the sensorelement 10. As shown in FIG. 5, in the present embodiment, proceduresfor manufacturing the sensor element 10 are roughly as follows: theelement base 1 including the ceramic body 101 as a laminated body of aplurality of solid electrolyte layers is manufactured using a knowngreen sheet process (step Sa), and then the outer leading-end protectivelayer 2 is fixed to the element base 1 to form the heat insulating space4 (step Sb). Accordingly, the ranges of the zones A, B, and C aresupposed to be already known.

At the manufacture of the element base 1, a plurality of blank sheets(not illustrated) being green sheets containing the oxygen-ionconductive solid electrolyte, such as zirconia, as a ceramic componentand having no pattern formed thereon are prepared first (step S1).

The blank sheets have a plurality of sheet holes used for positioning inprinting and lamination. The sheet holes are formed to the blank sheetsin advance prior to pattern formation through, for example, punching bya punching machine when the sheets are in the form of the blank sheets.Green sheets corresponding to a portion of the ceramic body 101 in whichan internal space is formed also include penetrating portionscorresponding to the internal space formed in advance through, forexample, punching as described above. The blank sheets are not requiredto have the same thickness, and may have different thicknesses inaccordance with corresponding portions of the element base 1 eventuallyformed.

After preparation of the blank sheets corresponding to the respectivelayers, pattern printing and drying are performed on the individualblank sheets (step S2). Specifically, a pattern of various electrodes, apattern of the heater 150 and the insulating layer 151, a pattern of theelectrode terminals 160, a pattern of the main surface protective layers170, a pattern of internal wiring, which is not illustrated, and thelike are formed. Application or placement of a sublimable material forforming the first diffusion control part 110, the second diffusioncontrol part 120, the third diffusion control part 130, and the fourthdiffusion control part 140 is also performed at the time of patternprinting.

The patterns are printed by applying pastes for pattern formationprepared in accordance with the properties required for respectiveformation targets onto the blank sheets using known screen printingtechnology. A known drying means can be used for drying after printing.

After pattern printing on each of the blank sheets, printing and dryingof a bonding paste are performed to laminate and bond the green sheets(step S3). The known screen printing technology can be used for printingof the bonding paste, and the known drying means can be used for dryingafter printing.

The green sheets to which an adhesive has been applied are then stackedin a predetermined order, and the stacked green sheets are crimped underpredetermined temperature and pressure conditions to thereby form alaminated body (step S4). Specifically, crimping is performed bystacking and holding the green sheets as a target of lamination on apredetermined lamination jig, which is not illustrated, whilepositioning the green sheets at the sheet holes, and then heating andpressurizing the green sheets together with the lamination jig using alamination machine, such as a known hydraulic pressing machine. Thepressure, temperature, and time for heating and pressurizing depend on alamination machine to be used, and these conditions may be determinedappropriately to achieve good lamination.

After the laminated body is obtained as described above, the laminatedbody is cut out at a plurality of locations to obtain unit bodies(referred to as element bodies) eventually becoming the individualelement bases 1 (step S5).

Formation (application and drying) of a pattern that becomes the innerleading-end protective layers 180 on the element base 1 at completion isthen performed on each of the cut out element bodies (step S6).Formation of the pattern is performed using a paste prepared in advanceso that the inner leading-end protective layers 180 as desired areeventually formed.

Each of the element bodies on which the pattern that becomes the innerleading-end protective layers 180 has been formed is then fired at afiring temperature of approximately 1300° C. to 1500° C. (step S7). Theelement base 1 is thereby manufactured. That is to say, the element base1 is generated by integrally firing the ceramic body 101 made of thesolid electrolyte, the electrodes, the main surface protective layers170, and the inner leading-end protective layers 180. Integral firing isperformed in this manner, so that the electrodes each have sufficientadhesion strength in the element base 1.

After the element base 1 is manufactured in the above-mentioned manner,formation of the outer leading-end protective layer 2 accompanied byformation of the heat insulating space 4 is then performed on theelement base 1. In the present embodiment, the heat insulating space 4is formed using a sublimable vanishing material that disappears throughfiring (combustion). FIGS. 6A to 6F schematically illustrate specificprocedures for forming the heat insulating space 4 and the outerleading-end protective layer 2. FIGS. 6A to 6F illustrate an example offormation on one surface of the element base 1.

First, a pattern of a sublimable vanishing material that disappearsthrough firing (combustion) is formed in accordance with a range and theshape of the heat insulating space 4 eventually formed (step S11).

Specifically, as illustrated in FIG. 6A, a printing plate 301 having, asprinting ranges, the ranges demarcated as the zones A and C is preparedto correspond to each of the surfaces of the element base 1 on the sideof the one end portion E1, and is disposed on the inner leading-endprotective layers 180 as shown by an arrow AR1.

As illustrated in FIG. 6B, the printing plate 301 includes a screen meshportion 301 a having an opening corresponding to the shape of the heatinsulating space 4 and a supporting portion 301 b to be disposed on theinner leading-end protective layers 180 while holding the screen meshportion 301 a in a tensioned state. More particularly, the supportingportion 301 b is formed to form, when disposed on the inner leading-endprotective layers 180, a gap 301 c at a predetermined angle α with theinner leading-end protective layers 180 at a location of the end portionof the heat insulating space 4 after formation of the heat insulatingspace 4. To obtain the end portion angle θ of 5° to 15°, it is enoughthat the angle α is 3° to 13°.

When the printing plate 301 is placed in this state, a squeegee 302 ismoved as shown by an arrow AR2 in the state of a vanishing materialpaste 5 prepared in advance being disposed on the screen mesh portion301 a to sequentially form a vanishing material pattern 5 a, asillustrated in FIG. 6C. The vanishing material pattern 5 a is eventuallyformed to extend to the gap 301 c as illustrated in FIG. 6D. The gap 301c may not necessarily completely be filled with the vanishing materialpattern 5 a, and the gap 301 c may remain as illustrated in FIG. 6D aslong as the end portion angle θ in the heat insulating space 4eventually obtained has a desired value.

When the vanishing material pattern 5 a is formed, the printing plate301 is removed, and the vanishing material pattern 5 a is driedappropriately. After formation and drying of the vanishing materialpattern 5 a on all the side surfaces and the leading end surface 101 eof the element base 1, slurry containing a material for forming theouter leading-end protective layer 2 is thermal sprayed onto the elementbase 1 on which the vanishing material pattern 5 a has been formed at aformation target location of the outer leading-end protective layer 2(step S12). FIG. 6E illustrates the state after thermal spraying. Thatis to say, a thermal sprayed film 2 a containing the material forforming the outer leading-end protective layer 2 is formed to cover thevanishing material pattern 5 a by thermal spraying.

The element base 1 on which the vanishing material pattern 5 a and thethermal sprayed film 2 a have been formed is then fired at a firingtemperature of approximately 300° C. to 600° C. (step S13). As a result,the vanishing material pattern 5 a sublimates and disappears, and, asillustrated in FIG. 6F, the heat insulating space 4 is formed at thelocation where the vanishing material pattern 5 a had been formed, andthe outer leading-end protective layer 2 is formed as a result that anorganic component volatilizes from the thermal sprayed film 2 a. Thesensor element 10 is thereby obtained.

The sensor element 10 thus obtained is housed in a predeterminedhousing, and built into the body, which is not illustrated, of the gassensor 100.

Second Embodiment

The configuration of the sensor element for securing water resistance byinterposing the heat insulating space between the outer leading-endprotective layer and the element base while suppressing delamination anddetachment of the outer leading-end protective layer is not limited tothat shown in the first embodiment. In the present embodiment, aconfiguration of a sensor element 20 heated in accordance with atemperature profile shifted to a lower temperature side compared withthe sensor element 10 according to the first embodiment is described.

FIG. 7 is a sectional view taken along the longitudinal direction of thesensor element 20 according to the second embodiment of the presentinvention. The sensor element 20 has components similar to those of thesensor element 10 according to the first embodiment except for somecomponents. The similar components thus bear the same reference signs asthose in the first embodiment, and detailed description thereof isomitted below.

As with the sensor element 10 according to the first embodiment, thesensor element 20 is used as a main component of the gas sensor 100,under operation control of the pump cells and the heater 150 performedthrough control of the pump cell power supply 30 and the heater powersupply 40 performed by the controller 50, although it is not illustratedin FIG. 7. Thus, in a case where the target of measurement of the gassensor 100 is NOx in the measurement gas, operation of the pump cellsand the heater 150 of the sensor element 20 is controlled throughcontrol of the pump cell power supply 30 and the heater power supply 40performed by the controller 50, and the NOx concentration in themeasurement gas is calculated by the controller 50 based on the linearrelationship between the current value (NOx signal) of the pump currentIp2 flowing through the measurement pump cell P3 under the control andthe concentration of decomposed NOx.

As illustrated in FIG. 7, the sensor element 20 includes, in place ofthe outer leading-end protective layer 2 of the sensor element 10, anouter leading-end protective layer (first leading-end protective layer)12 fixed to the element base 1 in a different manner from the outerleading-end protective layer 2. Specifically, the outer leading-endprotective layer 12 of the sensor element 20 is similar to the outerleading-end protective layer 2 of the sensor element 10 in that the heatinsulating space 4 is interposed between the outer leading-endprotective layer and the side surfaces of the element base 1. The outerleading-end protective layer 12, however, is different from the outerleading-end protective layer 2 in that an end surface portion 204 isfixed to the leading end surface 101 e of the element base 1 on the sideof the one end portion E1 of the element base 1, as the end surfaceportion 203 of the outer leading-end protective layer 2 is separatedfrom the element base. The heat insulating space 4 existing in thesensor element 20 is thus only the first space 4 a interposed betweenthe outer leading-end protective layer 12 and the side surfaces of theelement base 1, and the second space 4 b interposed in the sensorelement 10 is not present. Since the outer leading-end protective layer12 is a porous layer, introduction of the measurement gas into theelement base 1 (ceramic body 101) through the gas inlet 105 is performedwithout any problems.

That is to say, the outer leading-end protective layer 12 of the sensorelement 20 according to the present embodiment is fixed to the elementbase 1 in the fixed portion 201 having a band shape sequentially alongthe side surfaces of the element base 1 and the end surface portion 204.

As with the outer leading-end protective layer 2 of the sensor element10, the portion in which the fixed portion 201 of the outer leading-endprotective layer 12 and the element base 1 (inner leading-end protectivelayers 180) are in contact with each other is preferably 10% or more, inarea, of the total range in which the outer leading-end protective layer12 surrounds the element base 1. Detachment of the outer leading-endprotective layer 12 as a whole is less likely to occur compared with theouter leading-end protective layer 2 due to fixing to the end surfaceportion 204, but a fixed area ratio of less than 10% is not preferableas with the outer leading-end protective layer 2 as adhesion strength ofthe fixed portion 201 cannot be secured. On the other hand, the upperlimit of the fixed area ratio is determined in accordance with theminimum formation range of the heat insulating space 4 meeting thedesired condition, and may have a smaller value than that in the case ofthe sensor element 10.

Furthermore, the end portion angle θ is preferably similar to that ofthe sensor element 10 in terms of securing adhesion strength of thefixed portion 201. That is to say, the end portion angle θ is preferably5° to 15°.

The sensor element 20 having a configuration as described above can bemanufactured in a similar manner to the sensor element 10 according tothe first embodiment as described based on FIG. 5 and FIGS. 6A to 6Fexcept that, due to the difference in shape of the heat insulating space4 and the outer leading-end protective layer 12 eventually formed, thevanishing material pattern 5 a is not formed on the leading end surface101 e of the element base 1, and the thermal sprayed film 2 a is formedto be in contact with the leading end surface 101 e.

The difference between the sensor element 10 and the sensor element 20in presence or absence of the interposed second space 4 b corresponds tothe difference between them in temperature profile when the gas sensor100 is in use. As described above, the sensor element 20 according tothe present embodiment is assumed to be used in accordance with thetemperature profile shifted to the lower temperature side compared withthe sensor element 10 according to the first embodiment. Description ismade on this point based on FIG. 8. FIG. 8 is a diagram for moreparticularly describing specific placement locations of the outerleading-end protective layer 12 and the heat insulating space 4, and thesignificance thereof, similarly to FIG. 3.

In the sensor element 20, placement of the outer leading-end protectivelayer 12 and the heat insulating space 4 is determined based on zonesdividing the element base 1 as in the sensor element 10. As illustratedin FIG. 8, the sensor element 20 has the zones A, B, and C as with thesensor element 10. Definitions of these zones are the same as those inthe sensor element 10. That is to say, the zone A is the region at leastincluding the range from the first internal chamber 102 to the secondinternal chamber 103, and heated by the heater 150 to a temperature of500° C. or more when the gas sensor 100 is in use. The zone B is theregion starting at the end portion of the fixed portion 201 in which theouter leading-end protective layer 12 is fixed to the inner leading-endprotective layers 180 on the side of the one end portion E1, and endingat the other end portion E2 of the element base 1, and maintained at500° C. or less when the gas sensor 100 is in use. The zone C is theregion between the zones A and B in the longitudinal direction of theelement base 1.

While the zone A reaches the gas inlet 105 in the sensor element 10illustrated in FIG. 3, a predetermined range extending from the gasinlet 105 is classified as a zone D different from the zone A in thesensor element 20 illustrated in FIG. 8.

The zone D is a region maintained at 500° C. or less when the gas sensor100 is in use on the side of the one end portion E1 of the sensorelement 20. In other words, when the gas sensor 100 including the sensorelement 20 is in use, the sensor element 20 is heated by the heater 150provided inside the sensor element 20 so that the temperature profile inwhich the zone D is formed in addition to the zones A to C is achieved.

In the sensor element 20, the heat insulating space 4 (first space 4 a)is inevitably present at least around the portion of the element base 1belonging to the zone A as in the sensor element 10. Thus, once theportion belonging to the zone A and heated to a high temperature of 500°C. or more is exposed to water when the gas sensor 100 is in use, localand abrupt temperature reduction occurs in the water-exposed portion,but thermal shock caused by the temperature reduction in thewater-exposed portion does not occur in the element base 1. This isbecause the side surface portion 202 of the outer leading-end protectivelayer 12 is not in contact with the element base 1 and the heatinsulating space 4 (first space 4 a) having a large heat capacity isinterposed between them.

This is also as in the sensor element 10 that, even if water dropletsadhere to the portion in which the temperature is 500° C. or less whenthe gas sensor 100 is in use, abrupt temperature reduction hardlyoccurs, and thus thermal shock that may cause the water-induced crackinghardly occurs. In the sensor element 20, such a portion in which thetemperature is 500° C. or less during use is present not only in thezone B on the side of the other end portion E2, but also in the zone Don the side of the one end portion E1.

A suitable range of the thickness and the porosity of the outerleading-end protective layer 12 is similar to that of the outerleading-end protective layer 2 of the sensor element 10. The thicknessof the heat insulating space 4 is also similar to that of the sensorelement 10.

FIG. 9 illustrates an example of the relationship between theconfiguration of the sensor element 20 and the temperature profile ofthe sensor element 20 when the sensor element 20 is heated by the heater150 in accordance with a predetermined control condition when the sensorelement 20 is in use. The temperature profile shown in FIG. 9 isobtained by measuring the surface temperature on the pump surface of thesensor element 20 along the longitudinal direction of the element, andplotting it with the location of the leading end surface 101 e on theside of the one end portion E1 as the origin. Thermography is used tomeasure the surface temperature.

In the example illustrated in FIG. 9, a range extending from the leadingend of the element (one end portion E1) by a distance L3 is the zone D,and a range adjacent to the range and extending from the location of thedistance L3 to the location of the distance L1 is the zone A, incontrast to the case of FIG. 4. A range separated from the leading endof the element by the distance L2 or more is the zone B.

Also in the sensor element 20, having only to specify the zones andprovide the outer leading-end protective layer 12 so that the heatinsulating space 4 is formed in accordance with the ranges of the zonesat the manufacture of the sensor element 20, the heat insulating space 4comes to exist around the region (i.e., the zone A) every time heated bythe heater 150 to a temperature of 500° C. or more during use after themanufacture.

As in the sensor element 10, as for numerous sensor elements 20manufactured under the same condition, such as sensor elements 20industrially produced in large quantities, having only to specify thetemperature profile for a sensor element 20 extracted as a sample, andto demarcate the ranges of the zones A, B, C, and D based on thetemperature profile, a condition for forming the outer leading-endprotective layer 12 can be determined, based on the results, for allsensor elements 20 manufactured under the same condition withoutactually specifying the temperature profiles for all the sensor elements20. That is to say, it is not necessary to actually obtain thetemperature profiles for all the sensor elements 20, and demarcate theranges of the zones A, B, C, and D based on the results.

In other words, it can be said that, for the sensor elements 20manufactured under the same condition as described above, the region tobe coped with water-induced cracking of the element base 1 is specifiedin advance in accordance with setting of the control condition of theheater 150, as in the sensor element 10. In the case of FIGS. 8 and 9,the zone A corresponds to the region. The sensor element 20, however,differs from the sensor element 10 in that such a region exists only ina part of the side surfaces of the element base 1. It can be said thatthe outer leading-end protective layer 12 surrounds a predeterminedrange of the element base 1 on the side of the one end portion E1 sothat the heat insulating space 4 is interposed between the region to becoped with water-induced cracking and the outer leading-end protectivelayer 12. In this case, the outer leading-end protective layer 12 isfixed to the element base 1 (to the inner leading-end protective layers180) in the water-induced cracking not occurring region on the sidesurfaces of the element base 1 as in the sensor element 10. In the caseof FIGS. 8 and 9, the zone B corresponds to the region. The sensorelement 20, however, differs from the sensor element 10 in that theouter leading-end protective layer 12 is further fixed to the leadingend surface 101 e of the element base 1.

Also in a case where the temperature on the side of the one end portionE1 becomes 500° C. or less as in the sensor element 20 of FIG. 9, theouter leading-end protective layer 2 may be provided so that the secondspace 4 b is interposed between the outer leading-end protective layer 2and the element base 1 as in the sensor element 10 according to thefirst embodiment. This is because the heat insulating space 4 is stillpresent around the zone A.

As described above, in the present embodiment, the outer leading-endprotective layer as the porous layer is provided at least around theregion to be coped with water-induced cracking specified in advance andincluding the range from the first internal chamber to the secondinternal chamber of the element base of the sensor element included inthe gas sensor so that the heat insulating space is interposed betweenthe outer leading-end protective layer and the element base as in thefirst embodiment. The sensor element having great water resistance canthereby be achieved.

Furthermore, the inner leading-end protective layers having a largerporosity than the outer leading-end protective layer are provided on theouter periphery of the element base, and the outer leading-endprotective layer is fixed to the inner leading-end protective layers inthe water-induced cracking not occurring region specified in advance. Inaddition, the outer leading-end protective layer is fixed to the elementbase in the leading end portion of the element. Delamination and,further, detachment of the outer leading-end protective layer canthereby more surely and suitably be suppressed compared with the firstembodiment.

<Modifications>

The above-mentioned embodiments are targeted at a sensor element havingthree internal chambers, but the sensor element may not necessarily havea three-chamber configuration. That is to say, the configuration inwhich the inner leading-end protective layers having a large porosityare provided on outermost surfaces of the element base on the side ofthe end portion at least including the gas distribution part, and,further, the outer leading-end protective layer as the porous layerhaving a smaller porosity than the inner leading-end protective layersis provided outside the inner leading-end protective layers so that the(heat insulating) space is interposed between the outer leading-endprotective layer and the portion of the element base in which thetemperature becomes 500° C. or more during use is applicable to a sensorelement having one internal chamber or two internal chambers.

In the above-mentioned embodiments, the region heated to a temperatureof 500° C. or more during use is set to the region to be coped withwater-induced cracking on the premise of the configuration of the sensorelement illustrated in FIG. 2 or 7, but the heating temperature of theregion considerable as a target of the region to be coped withwater-induced cracking may vary depending on the configuration of thesensor element.

Examples

(Test 1)

As the sensor element 10 according to the first embodiment, six types ofsensor elements 10 (Examples 1 to 6) having different combinations ofthicknesses of the first space 4 a of the heat insulating space 4 andthicknesses of the outer leading-end protective layer 2 (thicknesses ofthe side surface portion 202 and the end surface portion 203) weremanufactured, and a test of water resistance was conducted on them.

As comparative examples, a sensor element (Comparative Example 1) inwhich the outer leading-end protective layer 2 as a whole adhered to theelement base 1 without the heat insulating space 4 being interposedtherebetween and a sensor element (Comparative Example 2) in which theouter leading-end protective layer 2 was not provided to expose theelement base 1 were manufactured, and a similar test was conducted onthem.

Table 1 lists the presence or absence of the first space 4 a, thethickness of the first space 4 a, the thickness of the outer leading-endprotective layer 2, and the results of determination in the waterresistance test for each sensor element. The element bases 1 of all thesensor elements were manufactured under the same condition. The sensorelements according to Examples 1 to 6 were each set to have a fixed arearatio of 30%, an end portion angle θ of 10°, and a thickness of thesecond space 4 b identical to the thickness of the first space.

TABLE 1 THICKNESS OF OUTER PRESENCE OR THICKNESS LEADING-END ABSENCE OFOF FIRST PROTECTIVE DETERMI- LEVEL FIRST SPACE SPACE [μm] LAYER [μm]NATION 1 EXAMPLE 1 PRESENT 100 300 ⊚ EXAMPLE 2 PRESENT 50 150 ⊚ EXAMPLE3 PRESENT 30 200 ⊚ EXAMPLE 4 PRESENT 20 200 ◯ EXAMPLE 5 PRESENT 150 200⊚ EXAMPLE 6 PRESENT 50 100 ◯ COMPARATIVE ABSENT — 300 X EXAMPLE 1COMPARATIVE ABSENT — — X EXAMPLE 2

The water resistance test was conducted by the following procedures:First, the heater 150 was energized to heat the sensor element 10 sothat a temperature profile in which a maximum temperature in the zone Awas 800° C., and the temperature in the zone B was 500° C. or less wasobtained. In the temperature profile, the range from the gas inlet 105to the third internal chamber 104 in the longitudinal direction of theelement belonged to the zone A.

While the heating state was maintained, the pump cells and, further, thesensor cells of the sensor element were operated in ambient atmosphereto perform control so that oxygen concentration in the first internalchamber 102 was maintained at a predetermined constant value to therebyobtain a situation in which a pump current Ip0 in the main pump cell P1was stabilized.

Under the situation, a predetermined amount of water was dropped ontothe side surface portion 202 of the outer leading-end protective layer 2belonging to the zone A (onto a corresponding portion of the elementbase 1 in Comparative Example 2), and whether a change of the pumpcurrent Ip0 before and after dropping exceeded a predetermined thresholdwas determined. If the change of the pump current Ip0 did not exceed thethreshold, the amount of dropped water was increased to repeat thedetermination. The amount of dropped water when the change of the pumpcurrent Ip0 eventually exceeded the threshold was defined as a crackingoccurring dropped water amount, and water resistance or a lack thereofwas determined based on the magnitude of a value of the crackingoccurring dropped water amount. Determination in this manner wasreferred to as Determination 1. A maximum value of the amount of droppedwater was set to 25 μL.

In this test, the change of the pump current Ip0 was used as a criterionfor determining the occurrence of cracking in the element base 1. Thisutilizes such a causal relationship that, when cracking of the elementbase 1 occurs due to thermal shock caused by dropping (adherence) ofwater droplets onto the outer leading-end protective layer 2, oxygenflows into the first internal chamber 102 through a portion of thecracking, and the value of the pump current Ip0 increases.

Specifically, the sensor element was determined to have great waterresistance if the cracking occurring dropped water amount was 20 μL ormore. The sensor element was determined to have water resistance in arange allowable in practical use if the cracking occurring dropped wateramount was 10 μL or more and less than 20 μL. The sensor element wasdetermined to have insufficient water resistance in terms ofpracticality if the cracking occurring dropped water amount was lessthan 10 μL. The value of 10 μL is a maximum value of an evaluation valueof the amount of water exposure in a water exposure test disclosed inJapanese Patent Application Laid-Open No. 2015-87161. In Japanese PatentApplication Laid-Open No. 2016-48230, a case where cracking does notoccur with an amount of dropped water of 3 μL is determined as anexample. If the cracking occurring dropped water amount is 10 μL ormore, the sensor element is thus determined to have greater waterresistance than that of a conventional sensor element.

In the sensor element including the outer leading-end protective layer2, delamination of the outer leading-end protective layer 2 in the fixedportion 201 did not occur until cracking of the element base 1 occurred.

In Table 1, as the results of Determination 1, a double circle is markedfor the sensor element in which the cracking occurring dropped wateramount is 20 μL or more or cracking does not occur upon dropping of amaximum amount of water, a single circle is marked for the sensorelement in which the cracking occurring dropped water amount is 10 μL ormore and less than 20 μL, and a cross is marked for the sensor elementin which the cracking occurring dropped water amount is less than 10 μL.

According to the results shown in Table 1, the sensor elements inExamples 1 to 6 are each marked with the double circle or the singlecircle, whereas the sensor elements in Comparative Examples 1 and 2 areeach marked with the cross. It was determined that cracking occurred inthe sensor element in Comparative Example 1 with an amount of droppedwater of 5 μL to 9 μL. It was also determined that cracking occurred inthe sensor element in Comparative Example 2 with an amount of droppedwater of less than 1 μL.

The results shown in Table 1 indicate that the sensor element havinggreater water resistance than that of the conventional sensor elementcan be achieved by providing the outer leading-end protective layer asthe porous layer having a thickness of 150 μm or more and 600 μm or lessat least around the portion of the element base of the sensor elementincluded in the gas sensor heated to a high temperature of 500° C. ormore when the gas sensor is in use so that a heat insulating spacehaving a thickness of 30 μm or more and 150 μm or less is interposedbetween the outer leading-end protective layer and the element base asin the first embodiment, for example.

(Test 2)

A test was conducted to determine the influence, on water resistance, ofthe relationship between the region to be coped with water-inducedcracking and placement locations of the heat insulating space 4 and theouter leading-end protective layer 2. The test should originally beconducted by providing, for element bases heated in accordance with thesame temperature profile, the outer leading-end protective layers 2 atdifferent locations. In the following description, however, as for thesensor element according to the first embodiment, a plurality of sensorelements having a thickness of the heat insulating space 4 of 100 μm, athickness of the outer leading-end protective layer 2 of 300 μm, a fixedarea ratio of 30%, and an end portion angle θ of 10° manufactured underthe same condition were prepared, and heated by the heater 150 underintentionally different heating conditions to obtain four combinations(Examples 7 to 10) of temperature distributions in the sensor elementand placement locations of the heat insulating space 4 and the outerleading-end protective layer 2, for ease of preparation.

On the other hand, as for the sensor element according to the secondembodiment, one type of sensor element (Example 11) in which the outerleading-end protective layer 12 was provided so that the heat insulatingspace 4 did not include the second space 4 b was prepared. The sensorelement was set to have a similar thickness of the heat insulating space4, a similar thickness of the outer leading-end protective layer 12, asimilar fixed area ratio, and a similar end portion angle θ to those inExamples 7 to 10.

As comparative examples, a plurality of sensor elements in each of whichthe outer leading-end protective layer having a thickness of 300 μm as awhole adhered to the element base 1 without the heat insulating space 4being interposed therebetween were prepared, and heated by the heater150 under intentionally different heating conditions to obtain threetypes (Comparative Examples 3 to 5) of temperature distributions in thesensor elements.

Table 2 lists, for each sensor element, the presence or absence of thefirst space 4 a, the surface temperature of the sensor element in themiddle of a portion in which the heat insulating space 4 (first space 4a) is interposed between the side surface portion of the outerleading-end protective layer and the element base (“FIRST SPACEINTERPOSED PORTION TEMPERATURE” in Table 2), the surface temperature ofthe sensor element in the vicinity of the fixed portion of the outerleading-end protective layer on the side of the one end portion (“FIRSTSPACE NOT-INTERPOSED PORTION TEMPERATURE” in Table 2), the presence orabsence of the second space 4 b, the surface temperature on the side ofthe one end portion E1 (“ELEMENT LEADING END PORTION TEMPERATURE” inTable 2), and the results of determination in the water resistance test.For each of the sensor elements in comparative examples in which therewas no heat insulating space, the surface temperature of a correspondingportion was measured. The surface temperature was measured bythermography. The element bases 1 of all the sensor elements weremanufactured under the same condition.

TABLE 2 FIRST SPACE PRESENCE PRESENCE FIRST SPACE NOT- OR ELEMENT ORINTERPOSED INTERPOSED ABSENCE LEADING END ABSENCE PORTION PORTION OFPORTION OF FIRST TEMPERATURE TEMPERATURE SECOND TEMPERATURE DETERMI-DETERMI- DETERMI- LEVEL SPACE [° C.] [° C.] SPACE [° C.] NATION 1 NATION2 NATION 3 EXAMPLE 7 PRESENT 850 500 PRESENT 800 ◯ ◯ ◯ EXAMPLE 8 PRESENT700 350 PRESENT 650 ⊚ ⊚ ◯ EXAMPLE 9 PRESENT 750 450 PRESENT 700 ◯ ◯ ◯EXAMPLE 10 PRESENT 700 350 PRESENT 500 ⊚ ⊚ ⊚ EXAMPLE 11 PRESENT 650 350ABSENT 450 ⊚ ⊚ ◯ COMPARATIVE ABSENT 650 350 ABSENT 600 X ⊚ X EXAMPLE 3COMPARATIVE ABSENT 800 600 ABSENT 750 X X X EXAMPLE 4 COMPARATIVE ABSENT850 450 ABSENT 800 X ◯ X EXAMPLE 5

The water resistance test was conducted by the same procedures as thosein Test 1 except that water was dropped at three locations. The firstdropping location was the same as that in Test 1. The second droppinglocation was the fixed portion 201 (a corresponding portion of the outerleading-end protective layer in each of Comparative Examples 3 to 5).The third dropping location was the end surface portion 203 or 204 onthe side of the one end portion E1 (a corresponding portion of the outerleading-end protective layer in each of Comparative Examples 3 to 5).

Determination of water resistance or a lack thereof at each droppinglocation was similar to that in Test 1. In Table 2, determination at thefirst dropping location, determination at the second dropping location,and determination at the third dropping location are respectively shownas “DETERMINATION 1”, “DETERMINATION 2”, and “DETERMINATION 3”.

According to the results shown in Table 2, the sensor elements inExamples 7 to 11 are marked with the double circle or the single circlein each of Determination 1 to Determination 3, whereas the sensorelements in Comparative Examples 3 to 5 are marked with the cross exceptthat the sensor elements in Comparative Examples 3 and 5 arerespectively marked with the double circle and the single circle inDetermination 2.

The results indicate that good water resistance can be obtained in theportion in which the outer leading-end protective layer is provided as awhole in a case where the outer leading-end protective layer is providedat least in the portion of the sensor element in which the temperaturebecomes 500° C. or more during use so that the heat insulating space 4is interposed between the outer leading-end protective layer and theelement base 1, and the temperature in the fixed portion in which theouter leading-end protective layer is fixed to the element base ismaintained at 500° C. or less.

On the other hand, the results indicate that, in a case where the heatinsulating space 4 is not provided in the sensor element, waterresistance is secured in the portion in which the temperature ismaintained at 500° C. or less during use, but good water resistancecannot be obtained in the portion in which the temperature becomes ahigh temperature of 500° C. or more.

The above-mentioned results show the effectiveness of the heatinsulating space provided to correspond to the region to be coped withwater-induced cracking.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A sensor element for a gas sensor detecting apredetermined gas component in a measurement gas, said sensor elementcomprising: an element base including: an elongated planar ceramic bodymade of an oxygen-ion conductive solid electrolyte, and having a gasinlet at one end portion thereof; at least two internal chambers locatedinside said ceramic body, and communicating with said gas inlet underpredetermined diffusion resistance; at least two electrochemical pumpcells including an outer pump electrode located on an outer surface ofsaid ceramic body, at least two inner pump electrodes located to faceeach of said at least two internal chambers, and solid electrolyteslocated between said outer pump electrode and each of said at least twoinner pump electrodes, said at least two electrochemical pump cellspumping in and out oxygen between each of said at least two internalchambers and an outside; and a heater buried in a predetermined range ona side of said one end portion of said ceramic body; and a firstleading-end protective layer being porous, and surrounding a first rangeat least including a part from a leading end surface to two of said atleast two internal chambers closest to said gas inlet of said elementbase on the side of said one end portion, wherein a single heatinsulation space is interposed between said first leading-end protectivelayer and a portion of said element base in which said two of said atleast two internal chambers closest to said gas inlet are located and atleast includes a part along said leading end surface.
 2. The sensorelement according to claim 1, wherein said first leading-end protectivelayer has a thickness of 150 μm or more and 600 μm or less.
 3. Thesensor element according to claim 1, wherein said single heat insulatingspace has a thickness of 30 μm or more and 150 μm or less.
 4. The sensorelement according to claim 1, further comprising a second leading-endprotective layer being porous, having a larger porosity than said firstleading-end protective layer, and located on a whole side surface ofsaid element base at least in said first range, wherein an end portionof said first leading-end protective layer opposite said one end portionis a fixed portion in which said first leading-end protective layer isfixed to said second leading-end protective layer.
 5. The sensor elementaccording to claim 4, wherein said single heat insulating space ispresent over said first range surrounded by said first leading-endprotective layer as a whole.
 6. The sensor element according to claim 5,wherein a region where water-induced cracking is to be prevented isdefined in said element base, and said fixed portion is located in saidregion.
 7. The sensor element according to claim 6, wherein said fixedportion is located in a second range of said element base maintained at500° C. or less when said gas sensor is in use.
 8. The sensor elementaccording to claim 5, wherein said first leading-end protective layerhas a porosity of 15% or more and 30% or less, and said secondleading-end protective layer has a porosity of 30% or more and 50% orless.
 9. The sensor element according to claim 5, wherein a portion inwhich said fixed portion of said first leading-end protective layer isin contact with said second leading-end protective layer is 10% or moreand 50% or less, in area, of said first range.
 10. The sensor elementaccording to claim 4, wherein a region where water-induced cracking isto be prevented is defined in said element base, and said fixed portionis located in said region.
 11. The sensor element according to claim 10,wherein said fixed portion is located in a second range of said elementbase maintained at 500° C. or less when said gas sensor is in use. 12.The sensor element according to claim 4, wherein said first leading-endprotective layer has a porosity of 15% or more and 30% or less, and saidsecond leading-end protective layer has a porosity of 30% or more and50% or less.
 13. The sensor element according to claim 4, wherein aportion in which said fixed portion of said first leading-end protectivelayer is in contact with said second leading-end protective layer is 10%or more and 50% or less, in area, of said first range.
 14. The sensorelement according to claim 4, wherein said at least two internalchambers are three internal chambers, said at least two electrochemicalpump cells are three electrochemical pump cells including said outerpump electrode, three inner pump electrodes located to face each of saidthree internal chambers, and solid electrolytes located between saidouter pump electrode and each of said three inner pump electrodes, andpumping in and out oxygen between each of said three internal chambersand the outside, and said first range surrounded by said firstleading-end protective layer at least includes a part from said leadingend surface to said three internal chambers of said element base on theside of said one end portion.
 15. The sensor element according to claim1, wherein said at least two internal chambers are three internalchambers, said at least two electrochemical pump cells are threeelectrochemical pump cells including said outer pump electrode, threeinner pump electrodes located to face each of said three internalchambers, and solid electrolytes located between said outer pumpelectrode and each of said three inner pump electrodes, and pumping inand out oxygen between each of said three internal chambers and theoutside, and said first range surrounded by said first leading-endprotective layer at least includes a part from said leading end surfaceto said three internal chambers of said element base on the side of saidone end portion.